Many vessels in animals transport fluids from one bodily location to another. In some vessels, natural valves are positioned along the length of the vessel to permit fluid flow in a substantially unidirectional manner along the length of the vessel. For example, natural valves are particularly in mammalian veins of the lower extremities to prevent blood from pooling in the lower legs and feet during situations, such as standing or sitting, when the weight of the column of blood in the vein can act to prevent positive blood flow toward the heart. A condition, commonly known as “chronic venous insufficiency”, is primarily found in individuals where gradual dilation of the veins, thrombotic events, or other conditions prevent the leaflets of the native valves from closing properly. This leads to significant leakage of retrograde flow such that the valve is considered “incompetent”. Chronic venous insufficiency is a potentially serious condition in which the symptoms can progress from painful edema and unsightly spider or varicose veins to skin ulcerations. Elevation of the feet and compression stockings can relieve symptoms, but do not treat the underlying disease. Untreated, the disease can impact the ability of individuals to maintain their normal lifestyle. The mechanisms involved in the development of chronic venous disease are reviewed in John J. Bergan et al., “Chronic Venous Disease”, N Engl. J. Med. 2006; 355: 488-98.
To treat venous valve insufficiency, a number of surgical procedures have been employed to improve or replace the native valve, including placement of artificial valve prostheses. These efforts have met with limited success and have not been widely adopted as methods of treating chronic venous insufficiency. More recently, efforts have been directed towards finding a suitable self-expanding or radially-expandable artificial valve prostheses that can be placed using minimally invasive techniques, rather than requiring open surgery and its obvious disadvantages. Thus far, use of prosthetic venous valves has remained experimental only.
Prosthetic valves have been developed that use a support frame such as a stent. Frequently, a graft member is attached to the support frame and provides a valve function to the device. For example, the graft member can be in the form of a leaflet that is attached to a stent and movable between first and second positions. In a first position, the valve is open and allows fluid flow to proceed through a vessel in a first direction, and in a second direction the valve is closed to restrict fluid flow in a second, opposite direction. Examples of such prosthetic valves are described in commonly owned U.S. Pat. No. 6,508,833, filed Mar. 21, 2001, and U.S. Publication No. 2004/0186558, published Sep. 23, 2004. Another example of a prosthetic valve assembly, including a valve seat and a movable valve composed of a flexible member, is provided by U.S. Pat. No. 5,413,599, filed Dec. 13, 1999.
Other prosthetic valves are attached directly to the vessel wall and do not include a support frame. Examples of such frameless valves are described in commonly owned U.S. Publication No. 20060265053, published Nov. 23, 2006.
One problem limiting the use of prosthetic valves is the potential for thrombus formation, particularly within the valve pockets where low fluid flow rates can result in pooling. Another problem is the tendency of SIS or other non-synthetic valve leaflet material to thicken over time, particularly in regions where the leaflet is in contact with the vessel wall. Such thickening can result in the leaflet becoming less flexible and unable to open and close completely in response to fluid flow within the vessel. Synthetic or polymer valves may also become less flexible due to excess tissue growth, particularly within the valve pockets.